Electrolyte-mediated assembly of charged nanoparticles

Solutions at high salt concentrations are used to crystallize or segregate charged colloids, including proteins and polyelectrolytes via a complex mechanism referred to as salting-out. Here, we combine small-angle X-ray scattering (SAXS), molecular dynamics (MD) simulations, and liquidstate theory to show that salting-out is a long-range interaction, which is controlled by electrolyte concentration and colloid charge density. As a model system, we analyze Au nanoparticles coated with noncomplementary DNA designed to prevent interparticle assembly via Watson-Crick hybridization. SAXS shows that these highly charged nanoparticles undergo gas to face-centered cubic (FCC) to glass-like transitions with increasing NaCl or CaCl2 concentration. MD simulations reveal that the crystallization is concomitant with interparticle interactions changing from purely repulsive to a longrange potential well condition. Liquid-state theory explains this attraction as a sum of cohesive and depletion forces that originate from the interelectrolyte ion and electrolyte-ion-nanoparticle positional correlations. Our work provides fundamental insights into the ef fect of ionic correlations in the salting-out mechanism and suggests new routes for the crystallization of colloids and proteins using concentrated salts. © 2016 American Chemical Society.

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